Low-noise microwave tube



Jan. 20, 1959 'T. E. EVERHART EIAL 2,370,367

LOW-NOISE MICROWAVE TUBE 3 Sheets-Sheet 1 Filed Aug. 1. 1955 Q gag/giiii? ig lz g AMI/v1: 720% .6. fuzz/1x7, 6/1/15: ,6 5/1041,

Irma [K United States Patent O LOW-N OISE MICROWAVE TUBE Thomas E.Everhart, Santa Monica, and Charles K.

Birdsall, Menlo Park, Calif., assignors to Hughes Aircraft Company,Culver City, Calif., a corporation of Delaware Application August 1,1955, Serial No. 525,663

8 Claims. c1. sis-3.6

This invention relates to electron stream amplifiers, and moreparticularly to a low-noise traveling-wave tube.

Traveling-wave tubes generally comprise an evacuated envelope, aslow-wave structure disposed within the envelope for propagatingelectromagnetic waves at a velocity substantially less than the velocityof light, and an electron gun disposed at one end of the envelope forprojecting an electron stream in an interacting relationship with thewaves propagated along the slow-wave structure.

Traveling-wave tubes are known to amplify waves with in an extremelybroad band of frequencies and, for this reason, are considered to bevery useful in the amplification of microwaves. The broad spectrum noisefigure of a traveling-wave tube, however, is also normally relativelyhigh; therefore, selecting an input signal out of tube-noise isespecially diflicult in the broad-band amplification of signals having arelatively low input power.

It is, therefore, an object of the present invention to provide alow-noise microwave tube.

It is another object of the invention to provide means whereby the noiseon the slow-wave circuit due to the noise in the electron stream of atraveling-wave tube may be reduced.

It is another object of this invention to reduce the ratio of noisepower due to the beam to thermal noise power (kTB), or in other words,to reduce the noise figure, of a traveling-wave tube.

Briefly, in accordance with the present invention, use is made of theKompfner dip phenomenon which is a null in the amplitude of thetraveling-wave at.a stationary position along the helix, determined bythe frequency of R. F. energy in the traveling-wave, beam current, andvarious other parameters. At the Kompfner dip, all the input signalpower on the circuit has been transferred to the beam, hence the circuitmay have a discontinuity at this point, and while the noise powerimpressed on the circuit by the beam will be reflected, the input signalpower will not. In one embodiment of the present invention means areprovided at the point of the Kompfner dip to reflect waves of noiseinduced on the slow-wave structure by the noise on the electron streamback toward the electron gun. Attenuating means are provided toselectively attenuate these reflected waves whereby the stream may bedemodulated of noise to a maximum extent, so that the waves of noisefrequencies induced on the slow- ""wave structure may be substantiallyeliminated.

In another embodiment an auxiliary slow-wave structure is disposed aboutthe electron stream between the electron gun and a principal oramplifying slow-wave structure. Means are then provided at the after endof the auxiliary slow-wave structure to either reflect or absorb wavesof noise frequencies induced on the auxiliary slow-wave structure by thenoise. modulation on the electron stream.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and 2,870,367 Patented Jan. 20, 1959 advantagesthereof, will be better understood from the following descriptionconsidered in connection with the accompanying drawings in which severalembodiments of the invention are illustrated by way of example. It is tobe expressly understood, however, that the drawings are for the purposeof illustration and description only, and are not intended as adefinition of the limits of the invention.

Fig. 1 is a diagrammatic sectional view of a low-noise traveling-wavetube in which one embodiment of the present invention is illustrated;

Fig. 2 is a sectional view of a low-noise traveling-wave tube showing anadditional embodiment of the present invention; and

Figs. 3, 4, 5, 6, 7, 8, and 9 are abbreviated schematic diagrams offurther embodiments of the invention.

Referring to the drawings and particularly Fig. 1, a traveling-wave tube10 is shown comprising an evacuated envelope 12 which has an elongatedportion 14 at its right end and a relatively short enlarged portion 16at its left end. Within the enlarged portion 16 of the envelope 12, anelectron gun 18 is shown comprising a cathode 20 which is provided witha filament 22, a focusing electrode 24 and an accelerating anode 26.Filament 22 is heated with direct current which is provided by afilament source of potential 28 the negative side of which is connectedto cathode 20 through the negative side of filament 22. Focusingelectrode 24 is maintained at the same potential as that of cathode 20by an appropriate connection thereto. Focusing electrode 24 has afrustoconical configuration with an internal surface of revolutiondisposed at an angle of 67%. degrees from its axis of symmetry. Anode 26is maintained a few hundred volts positive with respect to cathode 20 bymeans of an accelerating source of potential 30, anode 26 beingconnected to a tap 32 on source of potential 30 the positive terminal ofwhich is grounded. Adjacent anode 26 is shown a noise re- I antenna lead42 to an output ferrule 44. A collector electrode 46 is positioned atthe right end of the elongated portion 14 of the envelope 12 tointercept the stream electrons. Input ferrule 34, antenna lead 36 andauxiliary helix 38 are maintained at a potential somewhat positive withrespect to that of the anode 26 by means of a connection toa tap 48 onaccelerating source of potential 30. Ferrule 44, antenna lead 42, andprincipal helix 40 are maintained at a potential somewhat positive withrespect to ground by a collector source of potential 50 which isconnected between collector 46 and ground. Ferrule 44 is'thus connectedto a tap 52 on collector source of potential 50. In this mannercollector 46 is maintained at a potential positive with respect to thatof output ferrule 44 in order to prevent secondary electrons, which maybe produced by the stream electrons impinging upon the collectorsurface, from reaching the output ferrule 44.

Auxiliary and principal helices 38 and 40, which serve as the slow-wavecircuit for the traveling-wave tube 10, preferably-are made of amaterial such as tungsten or molybdenum, the principal requirement beingthat they retain their form especially with respect to the pitches anddiameters. In accordance with the present invention, auxiliary helix 38is maintained at a potential for demodulating the stream produced by theelectron gun 18 in the band in which the tube 10 is intended to operate.Maximum demodulation ordinarily occurs when the poisease? tential of theauxiliary helix 38 is somewhat below that of the potential of principalhelix 40, principal helix 40 being maintained at a potential for maximumamplification of the wave launched along the auxiliary helix 38 to beamplified.

Shown disposed about the elongated portion 14 of the envelope 12adjacent to and to the right of input cavity 54 a helix 74! is shownwhich may be made of a ferromagnetic material known simply as ferritematerial. When the ferrite material is operated with a predetermined D.C. magnetic field, it may have unilateral wave propagation properties atfrequencies within the band of operation of the traveling-wave tube 110,that is, as a unilateral device .may attenuate waves propagated in onlyone direction. In the case of the instant invention, it is employed toattenuate waves propagated only in the direction toward the electron gunll8.

Disposed contiguous to the envelope 12 about the left end of principalhelix d there is shown a resistance termination 76 which is employed tosuppress oscillations within the tube caused by the reflection of wavespropagated along principal helix 40 at its opposite ends. The use ofresistive termination 76 is not absolutely necessary although it will befound that tube operation will ordinarily be much enhanced by its use.

The traveling-wave tube 10 is provided with an input matching cavity 54external to the evacuated envelope 12, having a coaxial input cable 56connected thereto and an output matching cavity 58 connected to acoaxial output cable 60. As previously mentioned, auxiliary andprincipal helices 38 and 40 are connected to ferrules 34 and 44 by leads36 and 42, respectively. Leads 36 and 42 are located parallel to theelectric fields excited within matching cavities 54 and 58. Matchingcavity 54 has a configuration of a rectangular toroid with a concentriccollar 62 disposed about and spaced from matching ferrule 34. An opening64 in the end plate of cavity 54 facing the left end of auxiliary helix38 allows the full length of lead 36 to be energized, and, in additionincreases the tendency of the electric fields produced in the cavity toafiect or modulate the flow of electrons in the stream. Cavity 58 issimilarly constructed having a corresponding concentric collar 66disposed about and spaced from matching ferrule 44, and an opening 68 inthe plate facing the right end of principal helix 430.

A center conductor 70 of input coaxial cable 56 extends through anaperture in the annular wall of cavity 54 and is connected to concentriccollar 62 while the outer conductor of cable 56 is bonded to theperiphery of the aperture. Likewise, the center conductor 72 of outputcoaxial cable 58 extends through an aperture in the annular wall ofcavity 58 and is bonded to the periphery of the aperture in the samemanner as before or viceversa. Cavities 54 and 58 are fabricated with aninner surface composed of a highly conductive material and are broadlyresonant so as not to limit the frequency of operation of thetraveling-wave tube 10. The configuration shown and described for thecavities 54 and 58 provides suitable impedance matching from auxiliaryand principal helices 38 and 40 to coaxial cables 56 and 60,respectively, over a range of frequencies such as, for example, 2000 to4000 megacycles per second.

A solenoid 78 is coaxially positioned symmetrically about the envelope12 and appropriate direct current is maintained in solenoid 78 by meansof a potential source such as battery 80 so as to produce an axialmagnetic field of the order of more than 1000 gauss to constrain theelectron stream produced by the gun 118 and to provide a D. C. fieldwithin the ferromagnetic helix 74 whereby waves of noise frequencieswithin the operating band of the tube 10 may be attenuated alongauxiliary helix 38 in the backward direction, or toward electron gun l8.

In the operation of the traveling-wave tube 10, electron gun 118projects an electron stream through auxiliary J5 in which it shown inFig. 1.

and principal helices 38 and 30 to collector 4d. The antenna lead 36connected between input ferrule 3d and 38 is then energized by an inputsignal arriving at input cavity 54 through input coaxial cable 56. Thesignal wave is thus launched along the auxiliary helix 36. Under certainspecified conditions well known in the art, a phenomenon known as theKompfner null or dip takes place at the end of the auxiliary helix 38,i. e., the circuit voltage representing the energy of the input wavepropagated along-the auxiliary helix 38 will be zero at the end of theauxiliary helix. The fact that the auxiliary helix 38 ends abruptly atits right end will not, therefore, reflect electromagnetic energy of theinput signal Wave. However, demodulation of the stream produced by theelectron gun 118 will occur within the auxiliary helix 38, and waves ofnoise frequencies within the operating band of the tube 10 will beinduced on the auxilairy helix and will be reflected at the right end ofauxiliary helix 38.

Due to its unilateral wave propagation properties, ferromagnetic helix74 does not impede the input signal modulation of the stream and doesnot impede the demodulation of the stream of noise originally existingin it. However, the stream will continue to be modulated as it passesbetween auxiliary helix 38 and principal helix 40 by the input signalalthough all the noise energy represented by waves induced on theauxiliary helix 38 will be reflected (or radiated) at its right end duesimply to its abrupt discontinuity. The noise energy propagated in thedirection of electron gun 18 will then be attenuated by the unilateralattenuation characteristics of the ferromagnetic helix 741.

The waves passing from auxiliary helix 38 to principal helix 30 areamplified within the helix 40; and waves reflected at the output end ofprincipal helix 40 are attenuated by the resistive termination 76 ofprincipal helix 40. At the end of principal helix 40, the amplifiedelectromagnetic waves in flowing along output antenna lead 42,connecting principal helix 40 to output ferrule 44, excite an electricfield in output matching cavity 58. This electric field then induces acorresponding output signal on the center conductor 72 of output coaxialcable 60.

The traveling-Wave tube of Fig. 1 is again shown in a diagrammaticsectional view in Fig. 2 with all of the same associated circuitry andstructure although some of the structure has been changed in position.First, the input matching ferrule 34 is connected over input antennalead 36 to principal helix 40 whereas auxiliary helix 38 is unconnectedand then terminated at its opposite ends except for the connection tothe tap 48 on accelerating source of potential 30. Input ferrule 34 islikewise disposed between input antenna lead 36 and auxiliary helix 38and not adjacent to anode 26 of gun 18 as shown in Fig. 1. On thecontrary, from the gun 18 in Fig. 2 there is shown disposed alongelongated portion lll of envelope 112 auxiliary helix 38, input ferrule3 3, input antenna lead 36, principal helix 40, output antenna lead 42,output matching ferrule 44, and collector 46. The position offerromagnetic helix 74 is likewise changed although it is stillpositioned about auxiliary helix 38. Ferromagnetic helix 74 is thuspositioned adjacent to and to the left of input matching cavity 54.

Resistive material 76 in Fig. 2 is positioned approximately at thecenter of principal helix 40 and disposed about a portion of theelongated portion 114 of the envelope 12. Resistive material 76 therebyserves the same purpose as described in connection with Fig. 1 althoughin Fig. 2 it is obvious that it is unnecessary to terminate either endof principal helix 40 because each end may be matched to the input andoutput coaxial cables 56 and 60 by appropriate means including input andoutput matching cavities 54 and 58. Solenoid 78 is disposed about theenvelope 12 in substantially the same position Solenoid 78 is employedto produce an axial magnetic field as before and, to this end, battery80 is also again connected to solenoid 78. The operation of thelow-noise tube in Fig. 2 is substantially the same as the operation ofthe tube 10 shown in Fig. 1 although the existence of the phenomenaknown as the Kompfner dip is not necessarily relied upon.

In the operation of the tube 10 in Fig. 2, an electron stream isprojected through auxiliary helix 38 and principal helix 40 from gun 18.Waves of noise frequencies within the operating band of the tube 10 arelaunched along auxiliary helix 38 when the electron stream is projectedthrough auxiliary helix 38. At the right end of auxiliary helix 38 thereis an abrupt discontinuity and therefore waves of noise frequencieswithin the operating band of the tube 10 will be reflected as before.Ferromagnetic helix 74, as before, has unilateral wave propagationproperties whereby maximum electromagnetic induction along the auxiliaryhelix 38 may take place as waves progress in a direction of electronflow and waves reflected at the right end of auxiliary helix 38 areattenuated as they are reflected back toward the electron gun 18. Alow-noise stream is thus projected through input ferrule 34 and thencemodulated by an input signal.

Resistive material 76 is placed at the Kompfner dip so as to furthereliminate, by attenuation, any noise energy while not affecting the waveenergy desired to be amplified.

Referring to Fig. 3, there is shown a third embodiment which is amodification of the embodiment of Fig. l in which the center section ofthe tube has been modified. Resistive attenuator 76 and ferrite helix 74are shown removed and replaced by a nonreciprocal attenuator 75 which,as one unit, performs both functions of the two replaced elements 76.and 74. Attenuator 75 extends to cover the forward end of helix 40 aswell as the after end of helix 38. In operation of this embodiment thenonreciprocal attenuator 75 in a conventional way attenuates only energytraveling in the backward direction, toward the electron gun, and,therefore, precludes oscillation in traveling-wave tube 10 withoutattenuating any forward energy and without affecting forward gun energyin any other way as by adding noise.

Referring to Fig. 4, there is shown a modification of the embodimentshown in Fig. 1 in which the auxiliary helix 38 is terminated by aresistive load 71 which absorbs and dissipates the noise energy inducedon auxiliaryhelix 38 from the electron stream instead of refleeting itand dissipating it in a nonreciprocal attenuator 74, as described inconnection with Fig. 1.

Referring to Fig. 5, there is shown another modification of theauxiliary helix 38 of Fig. 1 which is neither terminated nor surroundedby a nonreciprocal attenuator 74, but which has a discontinuity at theafter end of helix 38 and an absorptive resistive termination 73 on theforward end of the auxiliary helix 38. In operation, the discontinuityon the afterend of helix 38 causes the noise signals to be reflectedback toward the electron gun and they are absorbed in load 73.

Referring to Fig. 6, there is shown a further modification of theembodiment of Fig. 1 in which auxiliary helix 38 is terminated on theafter end by resistive load 71 and 'on' the forward end by resistiveload 73 in a manner to absorb forward traveling noise energy in resistor71 while absorbing any reflected noise energy in resistive load 73.

Fig. 7 shows a modification of the embodiment of Fig. 1 in which withoutan actual break in helix 40' there is placed a reflector 77 at theKompfner dip point while nonreciprocal attenuator 74 absorbs energyreflected thereby in the same manner as described in connection withFig. l. I

Fig. 8 shows another modification of the embodiment of Fig. 1 in whichthere is an actual D. C. break between helix 38 and helix 40 withresistive load 79 coupled to the after end of helix 38 and resistivetermination 81 coupled to the forward end of helix 40.

Fig. 9 is yet another modification of the embodiment 5 of Fig. 1 inwhich, as in Fig. 7, a reflector 77 is placed 10 and a resistivetermination 81-is provided for the input end of helix 40.

There have thus been shown embodiments of the present invention whichprovide a microwave tube having a low-noise figure by virtue of themeans disclosed for removing noise from the electron stream.

What is claimed is:

1. A low-noise traveling-wave tube comprising: an electron gun forproducing a stream of electrons; a slowwave structure disposed alongsaid stream of electrons for propagating a predetermined traveling-wavesignal therealong; means coupled to said slow-wave structure at theKompfner dip point on said slow-wave structure where substantially allthe energy of said predetermined traveling-wave signal has beentransferred to the stream of 5 electrons, for selectively deriving anddissipating undesired noise wave energy from said stream and from saidslow-wave structure while permitting desired wave energy to pass saidKompfner dip point substantially unaffected.

2. The traveling-wave tube as defined in claim 1 wherein the means forderiving and dissipating the noise wave energy from the stream includesa reflector and a nonreciprocal attenuator for dissipating noise energytraveling back toward the electron gun.

3. In a traveling-wave tube having a principal slowwave structure and anelectron gun for projecting an electron stream through the principalslow-wave structure, an auxiliary slow-wave structure disposed about thestream between the principal slow-wave structure and the electron gunimmediately adjacent to the principal 0 slow-wave structure to beelectromagnetically coupled thereto and being of such a length relativeto that of the principal slow-wave structure that the two slow-wavestructures electromagnetically constitute one slow-wave structure with adirect-current discontinuity between the two slow-wave structures, meansfor reflecting undesired electromagnetic noise wave energy towards theelectron gun from the end of said auxiliary slow-wave structure oppositethe electron gun, and means for selectively attenuating saidelectromagnetic waves propagated only toward the electron gun along saidauxiliary slow-wave structure.

4. In a low-noise traveling-wave tube having a principal helicalconductor and an electron gun for projecting an electron stream througha principal helix: an auxiliary 65 helical conductor disposed about thestream between the v principal helix and the electron gun immediatelyadjacent to the principal helix and electromagnetically coupled thereto,said auxiliary helical conductor having unconnected and unterminatedends for reflecting undesired electromagnetic noise energy, and aferromagnetic helical conductor disposed about said auxiliary helix toattenuate electromagnetic waves propagated along said auxiliary helicalconductor in the direction of the electron gun, whereby said noiseenergy may be substantially eliminated.

5. A low-noise traveling-wave tube comprising: a principal slow-wavestructure; an electron gun for projecting an electron stream throughsaid principal slow-wave structure; magnetic means for producing anaxial constraining.magnetic field along said electron stream; an

such a length relative to that of said principal slo structure that thetwo slow-wave structures electromagnetically constitute one slow-wavestructure with a direct-current discontinuity between'them; means forrefiecting electromagnetic Wave energy toward said electron gun from theend of said auxiliary slow-wave structure opposite said electron gun;and ferrite helix means disposed about said auxiliary slow-wavestructure for selectively attenuating electromagnetic waves propagatedin the direction of said electron gun along said auxiliary slow-wavestructure, said ferrite helix means being adapted to resolve from saidaxial constraining magnetic field a transverse component for producingsaid attenuating.

6. A low-noise traveling-wave tube comprising: a principal helix; anelectron gun for projecting an electron stream through said principalhelix; an auxiliary helical conductor disposed about said stream betweensaid electron gun and said principal helix and disposed immediatelyadjacent thereto and electromagnetically coupled thereto and being ofsuch a length relative to that of said principal helix that saidprincipal helix and said auxiliary helical conductor electromagneticallyconstitute one slow-wave structurewith a direct-current discon tinuityat their juncture, said auxiliary helical conductor having unconnectedand unterminated ends for reflecting electromagnetic energy; and aferromagnetic helix disposed about said auxiliary helical conductor toattenuate electromagnetic waves propagated along said auxiliary helicalconductor in the backward direction toward said electron gun.

7. The traveling-wave tube as defined in claim 6, wherein means arecoupled to the end of said auxiliary helical conductor adjacent saidelectron gun for modulating said electron stream with an electromagneticinput signal wave.

8. The traveling-wave tube as defined in claim 7, wherein resistivemeans are disposed about the end of said principal helix adjacent saidauxiliary helical conductor to prevent said traveling-wave tube fromoscillating because of the propagation of electromagnetic waves alongsaid principal helix reflected at its opposite ends.

References Cited in the file of this patent UNITED STATES PATENTS2,584,597 Landauer Feb. 5, 1952 2,602,148 Pierce .luly l, 1952 2,798,183Sensiper July 2, 1957 2,798,203 Robertson luly 2, 1957 OTHER REFERENCESArticle entitled The Microwave Gyrator," Bell Sys- 25 tern Tech. lour.for January 1952, pages 22 to 27.

